I found one mod other than INTEEN that allowed for teens to be romantically involved with Young Adult and Adult Sims. You can find that mod HERE This mod has not been updated since May 2016 and reports in the comments say it is broke.
I wanted to have an updated version of this mod because I get irritated when my two teens are romantic with each other and then one ages up before the other and then they go days or weeks with no romance because the other is still a teen.
There are 2 flavors of this mod. They can be used together:
Teen_Adult_Romance<–This only allows them to do romantic interactions NO Mess Around/Woohoo
Teen_Adult_WoohooMessAround<–This will allow for Mess Around/Woohoo<–REQUIRES Teen_Adult_Romance in order for the interaction to appear in the pie menu
This mod DOES NOT have teen pregnancy. If you want teen pregnancy then you can use my Risky Woohoo
Any mod that modifies the same XML files WILL CONFLICT